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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-30 03:57:31 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-30 03:57:31 +0000 |
commit | dc0db358abe19481e475e10c32149b53370f1a1c (patch) | |
tree | ab8ce99c4b255ce46f99ef402c27916055b899ee /tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs | |
parent | Releasing progress-linux version 1.71.1+dfsg1-2~progress7.99u1. (diff) | |
download | rustc-dc0db358abe19481e475e10c32149b53370f1a1c.tar.xz rustc-dc0db358abe19481e475e10c32149b53370f1a1c.zip |
Merging upstream version 1.72.1+dfsg1.
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs')
-rw-r--r-- | tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs | 37 |
1 files changed, 37 insertions, 0 deletions
diff --git a/tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs b/tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs new file mode 100644 index 000000000..f95496a6c --- /dev/null +++ b/tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs @@ -0,0 +1,37 @@ +// Test a case where variance and higher-ranked types interact in surprising ways. +// +// In particular, we test this pattern in trait solving, where it is not connected +// to any part of the source code. +// +// check-pass + +trait Trait<T> {} + +fn foo<T>() +where + T: Trait<for<'b> fn(fn(&'b u32))>, +{ +} + +impl<'a> Trait<fn(fn(&'a u32))> for () {} + +fn main() { + // Here, proving that `(): Trait<for<'b> fn(&'b u32)>` uses the impl: + // + // - The impl provides the clause `forall<'a> { (): Trait<fn(fn(&'a u32))> }` + // - We instantiate `'a` existentially to get `(): Trait<fn(fn(&?a u32))>` + // - We unify `fn(fn(&?a u32))` with `for<'b> fn(fn(&'b u32))` -- this does a + // "bidirectional" subtyping check, so we wind up with: + // - `fn(fn(&?a u32)) <: for<'b> fn(fn(&'b u32))` :- + // - `fn(&!b u32) <: fn(&?a u32)` + // - `&?a u32 <: &!b u32` + // - `?a: !'b` -- solveable if `?a` is inferred to `'static` + // - `for<'b> fn(fn(&'b u32)) <: fn(fn(&?a u32))` :- + // - `fn(&?a u32) <: fn(&?b u32)` + // - `&?b u32 <: &?a u32` + // - `?b: ?a` -- solveable if `?b` is inferred to `'static` + // - So the subtyping check succeeds, somewhat surprisingly. + // This is because we can use `'static`. + + foo::<()>(); +} |